1. Field of the Invention
The present invention is directed to a process for operating a display screen having a liquid crystal layer disposed between rows and columns of a conductor matrix with the liquid crystal layer being switched from a first optical state to a second optical state by applying a voltage higher than an upper threshold voltage U.sub.12, being switched from the second to the first optical state by applying a voltage lower than a lower threshold voltage U.sub.21 and being maintained in one of said optical states by applying a holding voltage U.sub.h with U.sub.21 &lt;U.sub.h &lt;U.sub.12.
2. Prior Art
A bistable liquid crystal matrix display and the method of addressing the various matrix points of the display is described in an article in Berichte der Bunsen-Gesellschaft, Vol. 78, No. 9, 1974, pages 912-914. In this article, the display utilizes a cholesterin liquid crystal layer having a positive, anisotropic dielectric constant with the cell having vertical wall orientations. As pointed out in the article, when a voltage U.sub.12 is applied, the liquid crystal assumes a homeotropic-nematic texture, and, when the voltage drops below a lower threshold U.sub.12, the liquid crystal assumes a focal conical orientation which is a light scattering state. Thus, when a cell is disposed between cross polarizers the liquid crystal layer is in a homeotropic state or orientation, the cell will not pass light; however, the focal conical orientation or state, which causes scattering and depolarization of the light in the cell, will enable light to pass through the cross polarizers.
As pointed out in the reference, the procedure or method for using the device was as follows: first all of the matrix points were brought into an "off" or homeotropic state by a high voltage pulse and then a holding voltage U.sub.h is connected or applied to maintain or conserve the "off" state. Thereafter, the rows are consecutively scanned with a voltage pulse of the magnitude U.sub.h and the matrix elements or points of the row, which points are to be switched to a light transmitting or "on" state, are subjected to a voltage at the lower threshold, which voltage is preferably zero, until the focal conical orientation or state is formed. Meanwhile, the voltage 2U.sub.h is connected to the remaining matrix elements of the row which are not being switched. Following the write-in step, a return is made to a voltage U.sub.h to maintain the layer between each of the matrix points of the row in the desired optical state or condition.
The process is referred to as a "two-phase write-in" which involves an erasure as the first phase and the write-in as the second phase. The process utilizes a relatively low circuitry outlay; however, since the images are alternately constructed row by row and then totally erased, an attractive representation is not achieved. If the entire matrix were no longer erased at a specific time, but the individual rows are erased directly prior to addressing step, the optical impression could be considerably improved. This type of operation does, in fact, produce virtually steady images which exhibit virtually no optical interference but since a row-wise or row-group-wise supply of the drive circuits with a connected supply voltage leads to an extremely high outlay in the drive component, this type of operation is obviously unsuitable for use with integrated circuits.
Investigations carried out in association with the present invention have indicated that in particular in the case with the liquid crystal displays having a high multiplex ratio and small image points, a periodic erasure is to be maintained. In fact, when the holding voltage is connected, the optical state at each image point becomes increasingly disturbed from the edge-in liquid crystal displays long disinclination lines gradually travel inward and the image point can only be reconverted into a clean "off" texture by a significantly high voltage pulse. In other words, the "off" texture is disturbed and will gradually change into an "on" texture.